What causes bud rot in cannabis
Bud rot is a fungal infection, and in most cannabis crops the culprit is Botrytis cinerea, the gray mold that also ruins strawberries, grapes, and greenhouse tomatoes. Spores do not need an invitation. They arrive on air, dust, clothing, clones, re-used pots, and soil. What they need is a plant surface that stays wet for hours and air that has stopped moving. Give them both inside a tight cola and a clean-looking flower can turn into a brown, mushy patch in three or four days.
how to prevent cannabis bud rot
So the honest answer has two parts. The pathogen has to be present, and it almost always is, indoors and out. Then the environment has to favor it. Growers who lose colas to mold rarely have a spore problem. They have a moisture and airflow problem.
Cannabis Bud Rot Treatment: Stop Botrytis Fast
The three conditions Botrytis needs
1. Free moisture on plant tissue
Botrytis spores will not germinate on a dry surface. They need a film of water, and they need it to last. Plant pathologists generally put the infection window at four to eight hours of continuous wetness, which is why a single evening of overhead watering or one foggy night outdoors can start something you will not see for a week.
2. Still air
Inside a dense canopy, transpiration from thousands of stomata has nowhere to go. Humidity climbs to 70, 80, even 90 percent in the pocket around the main cola while your wall hygrometer reads a comfortable 50. Mold loves that microclimate more than it loves the room average.
Cannabis Bud Rot Prevention Methods
3. Moderate, shifting temperatures
Botrytis grows fastest between roughly 59 and 75 degrees Fahrenheit. Cool nights outdoors, or a lights-off drop in a sealed room, push relative humidity toward dew point. Warm days and cool nights are the classic setup for gray mold in field hemp.
Where all that moisture comes from
- Rain, dew, and fog settling on outdoor flowers that never fully dry before nightfall
- Overhead watering, foliar feeding, and any spray applied late in the day
- Greenhouse condensation dripping from the roof onto the canopy
- Transpiration trapped by a canopy that was never thinned or defoliated
- Irrigation that keeps the root zone soaked and the whole room at high humidity
- Pots sitting in standing runoff water
How the fungus gets inside a bud
Botrytis is an opportunist. It usually needs a wound or a weakened entry point before it can colonize healthy tissue, and a cannabis flower gives it plenty of choices.
- Caterpillar feeding, especially corn earworm and hemp borers, which tunnel into colas and leave frass and damage behind
- Aphids, thrips, and mites creating tiny punctures and sticky honeydew
- Stems snapped during low-stress training or SCROG weaving
- Pruning cuts and wind damage on outdoor plants
- Contaminated clones, seeds, or potting media carried in from a previous run
Once inside, the fungus moves along the stem and through the flower from the middle out. That is why the outside of an infected cola can look fine while the inside is already slime.
Plant traits and grow habits that raise the risk
- Dense, indica-leaning flower structure that traps moisture between calyxes
- Heavy nitrogen late in flower, which pushes leafy, soft tissue that dries slowly
- Oversized colas that never dry out completely after a watering or a rain event
- Packed canopies with no lollipopping or inner defoliation
- Plants that were revegged, heat-stressed, or otherwise stalled
- Late-flower harvest pushed too far into wet autumn weather
Outdoor, greenhouse, and indoor causes differ
Outdoors
The weather decides most of it. Fall rains, morning dew, and humid nights are the big drivers, with caterpillar damage running a close second in much of the United States.
Greenhouse and high tunnel
Condensation is the enemy. Without active venting and horizontal air movement, humidity sits in the canopy overnight and the roof drips directly onto the buds.
Indoors
Late-flower relative humidity above about 60 percent, irrigation close to lights-off, and a canopy that was never opened up are the usual suspects. A failed dehumidifier during week seven can cost an entire crop.
Causes that get blamed too often
Bad genetics gets named a lot, but the same cut that molds in one room finishes clean in another. Airborne spores get blamed too, though they are present in nearly every grow space and always have been. Nutrients alone do not cause rot. Excess nitrogen matters because it changes tissue texture, not because it feeds the fungus directly.
Early signs that it has already started
- A single sugar leaf turning brown and curling while its neighbors stay green
- A dry-looking patch on the outside of a cola that feels soft when you squeeze it
- Gray fuzz or dusty spores near the base of the flower
- A stem that snaps easily and shows brown tissue inside
Matching fixes to causes
- Keep late-flower humidity under 55 percent and run oscillating fans above and below the canopy.
- Water the root zone only, early in the day, and never spray flowers in flower.
- Defoliate the interior and space plants so air moves through, not just over.
- Scout for caterpillars and borers weekly outdoors and treat before they tunnel.
- Sanitize rooms, tools, and pots between runs, and never clone from a plant that showed rot.
- Harvest on time. A week of extra ripening in wet weather is how good grows end up in the compost.
None of this requires a lab. It requires watching the two things mold cares about: how long your flowers stay wet, and whether the air around them keeps moving.